# [Pw_forum] Turbo TDDFT for a quantum dot

dario rocca roccad at gmail.com
Sat Jul 23 09:43:35 CEST 2016

Dear Sunghwan,
I have not used the code on systems with the characteristics you describe.
This is my guess: As far as the ground state of your quantum dot converges
without a smearing the turboTDDFT code can be used and will be stable.
While the independent electron polarizability of a 0 gap system diverges
for \omega-->0, the Hartree part in the TDDFT kernel will increase the
value of the gap and/or will "push" the oscillator strength to higher
energies.
Best,
Dario

On Fri, Jul 22, 2016 at 6:37 PM, SungHwan Choi <sunghwanchoi91 at gmail.com>
wrote:

> Dear all,
> I am checking whether turboTDDFT program is suitable for my project. My
> project is to calculate absorption spectrum of metallic quantum dot. I have
> two questions.
>
> 1) From other post, I read the fact that turboTDDFT cannot calculate a
> metallic system. Here, how do we define metallic system? just system that
> have no band gap? then for small metallic quantum dot can be a non-metallic
> system since it has small band gap (0.1~0.5eV). case that fractional
> occupation does not happen? then, we can control the occupation parameter
> not to make fractional occupation.
>
> 2) A metallic quantum dot has a size-dependency for band gap. Thus, if
> size become larger, then band gap become narrow. When turboTDDFT failed to
> predict an absorption spectrum? and do you know a numerical trend of
> artifact, which come from low band gap?
>
> By the way, does this limit come from algorithm or implementation of
> turboTDDFT? As far as I understood there is no limit of algorithm itself. I
> think there is some practical issues on there. Would you tell me where some
> troubles come up?
>
> Sincerely
> Sunghwan Choi
>
>
> ========================================
> Sunghwan Choi
> Ph.D candidate
> Computational Quantum Molecular Science Lab
> Department of Chemistry, KAIST, Republic of Korea
>
>
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